1. Technical Field of the Invention
The present invention relates to an optical angle detection apparatus that optically measures a tilt angle of a plane of an object. This optical angle detection apparatus is used to detect the tilt of a screen of, for example, a liquid crystal projector and then to correct trapezoidal distortion of an image projected on the screen.
2. Description of the Related Art
When an image is magnified and projected onto a screen in a direction normal to a front surface of the screen (i.e., a screen surface) using a liquid crystal projector or the like, the projected image is displayed without distortion. On the other hand, when an image is projected not in the direction normal to the screen surface, the displayed image is distorted. Such distortion is generally referred to as trapezoidal distortion. A method for correcting the trapezoidal distortion through image signal processing is known. This method negates trapezoidal distortion by performing digital processing on an image signal to give reversed distortion to the image. A method for detecting the tilt angle of a screen and automatically correcting trapezoidal distortion based on the detected tilt angle has been suggested, and examples thereof are described in Patent References 1 to 4.
[Patent Reference 1] Japanese Patent Application Publication No. 2003-280089
[Patent Reference 2] Japanese Patent Application Publication No. 2003-283963
[Patent Reference 3] Japanese Patent Application Publication No. 2004-093275
[Patent Reference 4] Japanese Patent Application Publication No. 2004-134908
An angle detection apparatus disclosed in Cited Reference 3 detects the tilt angle of a screen for a projector using a line type passive distance measurement unit. This apparatus increases measurement accuracy by performing calculation for measuring distances in a plurality of directions using the line type passive distance measurement unit. For example, the angle detection apparatus calculates four tilt angles (from horizontal) of the screen relative to the projector based on distances measured in five directions by the line type passive distance measurement unit and then obtains the average of the four tilt angles.
An angle detection apparatus disclosed in Patent Reference 4 will now be briefly described with reference to FIGS. 16-19. This conventional angle detection apparatus uses an ultrasonic sensor 100 and FIG. 16 shows reflection characteristics of the apparatus. In FIG. 16, “(1)” indicates a state where the ultrasonic sensor 100 is oriented in a direction perpendicular (normal) to a screen 20. In this state, an ultrasonic wave transmitted by the ultrasonic sensor 100 is reflected at a right angle by the screen 20 and then returns to the ultrasonic sensor 100. In FIG. 16, “(2)” indicates a state where the ultrasonic sensor 100 is oriented at a small angle with respect to the direction normal to the screen. In this state, since an ultrasonic wave transmitted by the ultrasonic sensor 100 is incident at a small oblique angle on the screen 20, the intensity of the ultrasonic wave, which has returned to the ultrasonic sensor 100, is reduced. Similarly to the state (2), in a state (3) of FIG. 16, the intensity of an ultrasonic wave, which has returned to the ultrasonic sensor 100, is further reduced since the ultrasonic wave is incident at a larger oblique angle on the screen 20 than the state (2).
FIG. 17 shows the relationship between the states. In FIG. 17, a horizontal axis represents a rotation angle of the ultrasonic sensor 100 and a vertical axis represents the level of a reflected ultrasonic wave. When the rotation angle of the ultrasonic sensor 100 oriented in the direction perpendicular to the screen 20 is set to 0°, the reflected ultrasonic wave level is in the shape of a parabolic curve centered at a rotation angle of 0° as shown in FIG. 17. As can be seen from this figure, the reflected ultrasonic wave level is maximized when the ultrasonic sensor 100 is oriented perpendicular to the screen 20.
This characteristic is utilized to detect the angle of the screen 20 relative to the projector 10. FIG. 18 shows the concept of such angle detection. As shown, if the screen 20 is tilted at an angle θ relative to the projector 10, the ultrasonic sensor 100 is oriented perpendicular to the screen 20 when the ultrasonic sensor 100 rotates at the angle θ relative to the projector 10.
FIG. 19 shows the relationship between the rotation angle of the ultrasonic sensor 100 and the reflected ultrasonic wave level in the case of FIG. 18. As shown in FIG. 19, the reflected ultrasonic wave level is maximized when the rotation angle of the ultrasonic sensor 100 is θ. Thus, the angle of the screen 20 relative to the projector 10 can be detected by determining the rotation angle of the ultrasonic sensor 100 at which the reflected ultrasonic wave level is maximized.
However, the method using the line type passive distance measurement unit disclosed in Patent Reference 3 has a problem in that the circuit configuration or the calculation is complicated, thereby increasing the costs. In addition, when the passive type distance measurement unit is used, distance measurement cannot be performed if an object for measurement is bright and has no contrast. Moreover, when the tilt angle of a screen is measured, the angle cannot be measured until a light source lamp of the projector is lit. Thus, the method has a problem with ease of use.
In the method described in Patent Reference 4, to detect the peak of the reflected wave, the calculation and control unit must be always active while the ultrasonic sensor rotates. In the method of Patent Reference 4, all reflected wave levels detected while the sensor rotates are obtained and the maximum level thereof is calculated. Thus, the method of Patent Reference 4 requires very complicated, highly accurate operations.